1. Technical Field of the Invention
The present invention relates generally to GaAsSb devices, and specifically to improving temperature performance in GaAsSb semiconductor lasers.
2. Description of Related Art
Vertical-Cavity Surface-Emitting Lasers (VCSELs), Edge Emitting Lasers (EELs) and other types of semiconductor light emitting devices, such as quantum cascade lasers and light emitting diodes (LEDs), are becoming increasingly important for a wide variety of applications, including optical interconnect systems, optical computing systems and telecommunications systems. For high-speed optical fiber communications, emission wavelengths in the 1.2 to 1.6 μm range are desired. Various approaches to fabricating semiconductor light emitting devices in the 1.2 to 1.6 μm range have included using InGaAsP lattice matched to InP, wafer bonding of AlAs/GaAs to InP-based materials, using thallium compounds and using antimony compounds.
Recently, materials based on arsenide-antimonide compounds have become promising candidates for 1.2 to 1.6 μm optoelectronic devices grown on gallium arsenide (GaAs) substrates. For example, heterostructures fabricated using GaAsSb/GaAs materials have significant potential advantages for fabricating 1.2 μm VCSEL's due to the compatibility of the materials with well developed GaAs/AlAs distributed Bragg reflectors and AlAs oxidation techniques.
To reach a wavelength of 1.2 μm, a highly strained GaAs1-xSbx quantum well of antimony concentration 0.3<x<0.4 is typically utilized with a type-II band alignment. The GaAsSb/GaAs type-II band alignment produces an optical transition that is indirect in real space, corresponding to recombination of electrons and holes across the interface between GaAs and GaAsSb. As compared with band-to-band (direct) transitions entirely within GaAsSb (type I band alignment), the indirect transition across the GaAs/GaAsSb interface requires less antimony concentration in the quantum well, resulting in reduced biaxial compression.
However, the electron confinement in type-II heterostructures is relatively weak compared to type-I heterostructures. As a result, the threshold current is especially temperature sensitive for type-II GaAsSb/GaAs heterostructures. The threshold current (Ith) is the minimal current required to establish the population inversion necessary for lasing, and can be expressed by the following temperature-dependent formula: Ith(T)=I0e(T/To). When a characteristic temperature (To) is large, the temperature dependence of the threshold current of the laser is small, and the stability of the laser during oscillation is higher, especially at high output power. GaAsSb quantum well lasers exhibit a characteristic temperature of approximately 45 K, as compared to To>100K for InGaAsN quantum well lasers. Thus, when a GaAsSb/GaAs laser is operating at high temperatures, the threshold current required to produce lasing can be higher than a circuit specification allows.
One suggested solution for improving electron confinement is to embed the GaAsSb quantum well in AlGaAs. For example, in an article by Braun et al., entitled “Strained InGaAs/GaPAsSb heterostructures grown on GaAs (001) for optoelectronic applications in the 1100-1130 nm range,” J. Appl. Phys. 88 (5), p. 3004 (2000), which is hereby incorporated by reference, the authors sandwiched the GaPAsSb/InGaAs quantum well region between two doped and graded AlGaAs cladding layers to improve temperature performance of the device. However, even for a relatively high aluminum concentration of about 40%, the electron confinement barrier relative to the GaAsSb conduction band edge is still rather modest. Therefore, what is needed is an GaAsSb material structure capable of emitting in the 1.2 to 1.6 μm range with improved temperature performance.